IMPROVING RADIOIMMUNOTARGETING OF TUMORS - VARIATION IN THE AMOUNT L6MAB ADMINISTERED, COMBINED WITH AN IMMUNOADSORPTION SYSTEM (ECIA)

Extracorporeal immunoadsorption (ECIA) is a new method for the selecti ve removal of circulating radiolabeled monoclonal antibodies (MAb) fro m plasma to increase the uptake in tumor versus normal tissues (T/N-ra tio). To ascertain whether the amount of MAb affects T/N ratios immedi ately and 24 h after ECIA, we used a rat model with two tumor sites-on e intramuscular (im) and one below the subrenal capsule (SR). Extracor poreal immunoadsorption was done with an avidin-agarose column after i njection of I-125-labeled biotinylated L6 MAb. The animals received 10 , 50 or 250 pg of L6 only (controls), or followed by ECIA. The efficac y of the procedure in removing plasma activity was 80-95%. For both tu mor sites, the highest T/N-ratios were obtained with 10 mu g L6. All T /N-ratios significantly improved for SR tumors by a factor ranging fro m 3.2 (lung) to 12.6 (bone marrow). The T/N-ratios were still elevated 24 h after ECIA. Injection of larger amounts of MAb, probably causing a higher degree of tumor saturation, will not necessarily improve the T/N ratio after ECIA.